A passive optical network (PON) is a point-to-multipoint network architecture in which unpowered optical splitters are used to enable a single optical fibre to serve multiple premises. A PON typically includes an Optical Line Terminal (OLT) at the service provider's central office connected to a number (typically 32-128) of Optical Network Terminals (ONTs), each of which provides an interface to customer equipment.
In operation, downstream signals are broadcast from the OLT to the ONTs on a shared fibre network. Various techniques, such as encryption, can be used to ensure that each ONT can only receive signals that are addressed to it. Upstream signals are transmitted from each ONT to the OLT, using a multiple access protocol, such as time division multiple access (TDMA), to prevent “collisions”.
A Wavelength Division Multiplexing PON, or WDM-PON, is a type of passive optical network in which multiple optical wavelengths are used to increase the upstream and/or downstream bandwidth available to end users. FIG. 1 is a block diagram illustrating a typical WDM-PON system. As may be seen in FIG. 1, the OLT 4 comprises a plurality of transceivers 6, each of which includes a light source 8 and a detector 10 for sending and receiving optical signals on respective wavelength channels, and an optical combiner/splitter 12 for combining light from/to the light source 8 and detector 10 onto a single optical fibre 14. The light source 8 may be a conventional laser diode such as, for example, a distributed feed-back (DFB) laser, for transmitting data on the desired wavelength using either direct laser modulation, or an external modulator (not shown) as desired. The detector 10 may, for example, be a PIN diode for detecting optical signal received through the network. An optical mux/demux 16 (such as, for example, an Arrayed Waveguide Grating—AWG—, or a Thin-Film Filter—TFF) is used to couple light between each transceiver 6 and an optical fibre trunk 18, which may include one or more passive optical power splitters (not shown).
A passive remote node 20 serving one or more customer sites includes an optical mux/demux 22 (which may, for example, also be an AWG or TFF) for demultiplexing wavelength channel (λ1 . . . λn) from the optical trunk fibre 18. Each wavelength channel is then routed to an appropriate PON 24 comprising one or more Optical Network Terminals (ONTs) 26 at respective customer premises. Typically, each ONT 26 includes a light source 28, detector 30 and combiner/splitter 32, all of which are typically configured and operate in a manner mirroring that of the corresponding transceiver 6 in the OLT 4.
Typically, the wavelength channels (λ1 . . . λn) of the WDM-PON are divided into respective channel groups, or bands, each of which is designated for signalling in a given direction. For example, L-band (1565-1625 nm) channels are typically allocated to downlink signals from the OLT 4 to each of the PONs 24, while C-band (1530-1565 nm) channels are allocated to uplink signals transmitted from each PON 24 to the OLT 4. In such cases, the respective optical combiner/splitters 12,32 in the OLT transceivers 6 and ONTs 26 are commonly provided as passive optical filters well known in the art.
The WDM-PON illustrated in FIG. 1 is known, for example, from “Low Cost WDM PON With Colorless Bidirectional Transceivers”, Shin, D J et al, Journal of Lightwave Technology, Vol. 24, No. 1, January 2006. With this arrangement, each PON 24 is served by a predetermined pair of wavelength channels, comprising an L-band channel for downlink signals transmitted from the OLT 4 to the respective PON 24, and a C-band channel for uplink signals transmitted from the respective PON 24 to the OLT 4. The MUX/DEMUX 16 in the OLT 4 directs couples the selected channels of each PON 24 to a respective one of the transceivers 6. Consequently, each transceiver 6 of the ONT is associated with one of the PONs 24, and controls uplink and downlink signalling between the ONT 4 and that PON 24. Each transceiver 6 and 26 in the OLT 4 and ONTs is rendered “colorless”, by using reflective light sources 8, 28, such as, reflective semi-conductor optical amplifiers; injection-locked Fabry-Perot lasers; reflective electro-absorptive modulators; and reflective Mach-Zehnder modulators. With this arrangement, each light source 8, 28 requires a “seed” light which is used to produce the respective downlink/uplink optical signals. In the system of FIG. 1, the seed light for downlink signals is provided by an L-band broadband light source (BLS) 32 via an L-band optical circulator 34. Similarly, the seed light for uplink signals is provided by a C-band broadband light source (BLS) 36 via a C-band optical circulator 38.
WDM-PONs suffer a limitation in that they are designed around a one-to-one connection paradigm. That is, each transceiver 6 of the OLT 4 communicates with the ONT(s) 26 of only one PON 24. However, it is desirable to also be able to broadcast analog signals to all of the ONT(s) 26. For example, it would be desirable to be able broadcast analog RF/video signals to subscribers through the WDM-PON infrastructure. Furthermore, it would be desirable to be able to provide this capability without compromising the performance of the WDM-PON or requiring active components within the network.